1. Field of the Invention
This invention relates generally to semiconductor light emitting devices, and more particularly, to III-nitride based light emitting diode devices with highly reflective ohmic contacts.
2. Description of Related Art
A xe2x80x9cIII-nitridexe2x80x9d material system is any combination of group III and group V elements, with nitrogen being the primary group V element, to form semiconductors used in the fabrication of electronic or optoelectronic devices. This material system includes, but is not limited to, GaN, AlGaN, AlN, GaInN, AlGaInN, InN, GaInAsN, and GaInPN. The III-nitride material system is suitable for the fabrication of light-emitting devices (LEDs) that generate light with photon energies from the ultra-violet to the red spectral wavelength regimes. These LEDs include light-emitting diodes and laser diodes.
A III-nitride LED typically includes epitaxial layers deposited upon a suitable growth substrate to form a p-n junction via growth techniques, e.g. organometallic vapor-phase epitaxy. There are some unique challenges in the fabrication of III-nitride semiconductor devices. Because III-nitride substrates are not commercially available, the epitaxial growth is forced to occur upon non-lattice-matched substrates, e.g. sapphire or SiC. The epitaxy-up orientation of the conventional III-nitride LED die, also called xe2x80x98junction upxe2x80x99 LEDs, requires that light be extracted out the top surface, i.e. out through the p-type III-nitride layers. But, the high resistivity of p-type III-nitride layers, e.g. GaN, requires that metallization be deposited on the p-type material surface to provide sufficient current spreading. Because such metals absorb light, a very thin p-electrode metallization (e.g., Ni/Au) is typically used to allow light to escape through the top surface. However, even these thin semi-transparent layers absorb a significant amount of light. Assuming a typical thickness of 100 xc3x85 of Au and neglecting Ni (which may be oxidized to form transparent NiOx), the amount of light absorbed in this semi-transparent p-electrode is xcx9c25% per pass at xcex=500 nm. At high current densities, the metallization thickness may need to be increased to maintain uniform current injection into the active region, and to avoid generating most of the light in the vicinity of the wirebond pad. Increasing the metal thickness increases light absorption and reduces the extraction efficiency of the device. Clearly, this tradeoff should be avoided in the design of III-nitride LEDs for operations at high current densities ( greater than 40 A/cm2, which is xcx9c50 mA into a xcx9c0.35xc3x970.35 mm junction area).
The light extraction efficiency of a light emitting diode (LED) is defined as the ratio of the LED""s external quantum efficiency to the LED""s internal quantum efficiency. The external efficiency of LEDs is strongly dependent on how efficiently light is coupled out of the semiconductor used to make the LED. Typically, the light extraction efficiency of a packaged LED is substantially less than one, i.e., much of the light generated in the LED""s active region never reaches the external environment.
The optical extraction efficiency for junction-up LEDs is limited due to optical absorption in the extended semi-transparent p-metallization. In particular, the use of reflective p-type contacts has been disclosed, as stated below. However, the p-type contacts are typically alloys or multi-layer structures and the n-type contacts consist of multi-layer structures containing a thin layer of Ti or V, covered with a thicker layer of Al or Au. In xe2x80x98junction upxe2x80x99 LEDs, the reflectivity of n-type contacts was of low concern as the absorption in the semi-transparent p-type contacts is very high.
Conventional GaN-based LEDs are fabricated in one of two ways. In one case light is mainly collected from the top side of the device where the epitaxial layers are formed. In this case, light emitted from the semiconductor must pass through a semi-transparent contact. This contact absorbs a significant fraction of the light generated in the material, lowering the efficiency of the LED. Alternatively, commercial LEDs are fabricated on SiC substrates and mounted so that one contact is on top of the chip and one is on the bottom. This configuration has the disadvantage that the SiC substrate absorbs a large fraction of the generated light. In both types of LEDs, the n- and p-type contacts are poor reflectors and they absorb significant fractions of the light generated by the device.
In FIG. 1, Nakamura et al., in U.S. Pat. No. 5,563,422, disclosed a typical prior art xe2x80x98junction upxe2x80x99 III-nitride LED employing a sapphire substrate. Undoped and doped III-nitride layers surround an active region. A non-planar device geometry is necessary where contact to both p and n regions occur on the same side (top) of the LED since the substrate is electrically insulating. Also, two wirebond pads are required on the top of the device. The n-side wirebond pad is also an Ohmic electrode for making electrical connection to the III-nitride epi layers. The high resistivity of the p-type III-nitride layers requires current spreading to be provided by a thin semi-transparent (partially absorbing) NiAu Ohmic electrode that is electrically connected to the p-type III-nitride layers. Light extraction efficiency is limited by the amount of surface area covered by this Ohmic electrode and by the bonding pads. The optical losses associated with the Ohmic and bondpad metal layers are accentuated by the light-guiding nature of the III-nitride materials (nxcx9c2.4) on the sapphire substrate (nxcx9c1.8). Moreover, the preferred electrodes are formed of a metallic material containing two or more metals, either alloyed together or in the form of a multi-layered structure. The preferred p-electrode is a multi-layer Ni/Au contact though the contact may be formed using combinations of gold (Au), nickel (Ni), platinum (Pt), aluminum (Al), platinum (Pt), tin (Sn), indium (In), chromium (Cr) and titanium (Ti). The n-type contact disclosed is also a multi-layer structure; preferably Ti/Al, Ti/Au or Ti/Al/Au.
Inoue et al., in EP 0 921 577 A1, disclosed a prior art III-nitride LED having an epitaxy-side down or inverted structure where the light escapes predominantly upwards through a superstrate, i.e. the sapphire growth substrate. The device design conserves active junction area and provides for the smallest possible die size. The p electrode is made of Ni and Au, which are quite absorbing to visible light. The n electrode is made of Ti and Au multi-layer film. Since this device lacks a highly reflective p-electrode metallization, it is limited in terms of light extraction efficiency and does not offer a significant improvement over the conventional (epitaxy-side up) device. Also, because the devices are small ( less than 400xc3x97400 xcexcm2) and use a small solder connection area to the package, they are limited in their light generating capability. Finally, this device suffers in efficiency from having guided light trapped within the III-nitride epi layers because of the low-refractive-index sapphire superstrate.
Kondoh et al., in EP 0 926 744 A2, disclosed a prior art inverted III-nitride LED using a sapphire superstrate. The p-type electrode is silver-based (i.e., a multi-layer electrode with silver as the first layer). The silver layer is very reflective to light and results in a device with higher light extraction efficiency compared to the device disclosed by Inoue et al. However, Ag adhesion to III-nitride material is poor. Upon annealing, Ag can conglomerate and destroy the integrity of the sheet ohmic contact behavior and the reflectivity. The n-type electrode is a conventional Ti/Al multi-layer electrode with 10 nm of Ti. The inclusion of the Ti layer promotes adhesion and reduces the contact resistance, although it also reduces the optical reflectivity of the contact. Since the device is relatively small ( less than 400xc3x97400 xcexcm2) and uses a small solder connection area to the package, it is limited in its light generating capability. Finally, this device suffers in efficiency from having guided light trapped within the III-nitride epi layers because of the low-refractive-index sapphire superstrate.
Mensz et al., in Electronics Letters 33 (24) pp.2066-2068, disclosed a prior art inverted III-nitride LED using a sapphire superstrate. This device employs bi-layer metal p-electrodes, Ni/Al and Ni/Ag, that offer improved reflectivity compared with Ni/Au. However, these bi-layer metal p-electrodes still have a relatively low reflectivity. The Ni layer of the p-electrode is used for the adhesion layer of the p-electrode. The n-electrode is a TiAl electrode, with an unspecified Ti thickness. There can be many advantages and reasons for using multi-layered metal p-electrodes (e.g., for adhesion to the semiconductor or to facilitate ohmic contacts). The drawback of using such additional layers is that they degrade the reflectivity of the electrode and hence the extraction efficiency. These devices also exhibited high forward voltages of 4.9 to 5.1V at 20 mA in 350xc3x97350 xcexcm2 devices. This yields a series resistance of xcx9c100xcexa9, which is more than three times higher than that of devices with good Ohmic electrodes. The high series resistance severely limits the power conversion efficiency. Since these devices are small ( less than 400xc3x97400 xcexcm2) and not mounted for low thermal resistance, they are limited in their light generating capability. Finally, these devices suffer in efficiency from having guided light trapped within the III-nitride epi layers because of the low-refractive-index sapphire superstrate.
Edmond et al., in WIPO WO96/09653, disclosed a vertical injection III-nitride LED on a conducting SiC substrate (i.e., an LED sandwiched between two ohmic contacts), shown in FIG. 2 with the ohmic contacts formed from Al, Au, Pt or Ni. A sapphire substrate may not be used in a vertical LED as sapphire is not conductive. A conductive buffer layer is required for Ohmic conduction from the III-nitride layers to the SiC substrate. The growth conditions required for a conductive buffer layer limits the growth conditions available for subsequent layers and thus restricts the quality of the III-nitride active region layers. Also, the conductive buffer layer may introduce optical loss mechanisms that limit light extraction efficiency. Furthermore, the SiC substrate must be doped to provide high electrical conductivity (xcfx81 less than 0.2 xcexa9-cm) for low series resistance. Optical absorption resulting from SiC substrate dopants limits the light extraction efficiency of the device. These conditions result in a trade-off between series resistance and light extraction efficiency and serve to limit the electrical-to-optical power conversion efficiency of the LED in FIG. 2.
The present invention is an inverted III-nitride light-emitting device (LED) with highly reflective ohmic contacts. Large area III-nitride LEDs increase the areal density of light generation and enable high-power operation. Inverted structure, or flip-chip III-nitride LEDs have been demonstrated to provide an increased extraction efficiency. As the surface area covered by the p-type electrode in flip-chip LEDs has increased, so has concern for the reflectivity of the p- and n-type electrodes. A large area ( greater than 400xc3x97400 xcexcm2) device has at least one n-electrode which interposes the p-electrode metallization to provide low series resistance. Both the n- and p-electrode metallization are opaque, highly reflective, Ohmic (specific contact resistance less than 10xe2x88x922 xcexa9cm2), and provide excellent current spreading. Highly reflective ohmic contacts enable enhanced optical extraction efficiency, especially for large-area flip-chip devices. Materials of choice are silver (Ag) and aluminum (Al) where the isotropic reflectivity values into n-GaN reach 97% and 87%, respectively. Light absorption in both the n- and p-electrodes at the peak emission wavelength of the LED active region is less than 25% per pass in each electrode.
This invention will be more fully understood in light of the following detailed description taken together with the accompanying drawings.